GASTEROPOD SWIMMERS. 129 



I have noticed that the Valvata, when in confinement, 

 will remain days shut up in this manner ; the Limnaeus stag- 

 nalis is less obstinate, but still a great alarmist ; while the 

 Physae, nearly allied to the Lymnaeus, and apparently less 

 defenceless, are only momentarily arrested by a touch, when 

 they again extend themselves and go on unhesitatingly. But 

 the Helicolimax Lamarckii a terrestrial species affords 

 quite an exception to the general rule, for " if disturbed or 

 irritated it only crawls the faster ; and if at rest and con- 

 tracted, it directly puts itself in motion on being touched or 

 disturbed."* The Nanina, an East Indian land mollusk 

 discovered by Mr. Benson, has the same habit as the Helico- 

 limax. f 



All Gasteropods are not confined, however, to crawl on 

 the solid bottom : many of them can ascend to the surface, 

 and make the water a liquid pavement, along which they 

 creep in the same manner as they do on land, with the dif- 

 ference only of having their body and shell in a reversed 

 position. The Aplysiae, and many of our nudibranchial 

 mollusca, may be seen crossing pools on our shore in this 

 way ; and there is reason to believe that all the marine naked 

 mollusca possess the faculty. J When I have confined a 

 number of the minute Rissoae, so common on our coasts, in a 

 glass of sea-water, some have very soon suspended themselves 

 from the surface ; but it is the freshwater snails (Mollusca 

 pulmonifera) which exhibit this not unremarkable mode of 

 progression in the most per- 

 fect manner. On a sum- g ' 21 ^ 

 mer's day any one may see 

 the Limnaei (Fig. 21) and 

 Planorbes thus traversing 

 the surface of ponds and 

 ditches in an easy undulat- 

 ing line ; or suspended there in luxurious repose, perhaps 



" To taste the freshness of heaven's breath, and feel 

 That light is pleasant, and the sunbeam warm." 



* Lowe in Zool. Journ. iv. 342. 



t Proc. Zool. Soc. 1834, p. 89. 



t Risso, 1'Europ. Me'rid. iv. 39, 47. 



Miiller says that in this position no motion of the foot is perceptible. 

 "In fluviatilibus nulla qm'dem undulatio percipitur ; ope tamen occultse 

 rotationis vel ignoto mechanismo nee lentius, quam terrestres, progrediun- 

 tur." Hist. Verm. ii. pref. xx. Lister had previously made the same 

 observation. Exercit. Anat. de Coch. 153. 



Quatrefages is of opinion that the progression of mollusks in this reversed 

 position on the surface of the water, cannot be made by any muscular 



K 



